Add a "Get principals for permission" operation in Document category

This operation accepts document,documents and produces document, documents
it has some parameters:

• permission: name of the permission for which we get the principals
• variable name: name of the context variable to which the operation adds principals list. If it is null the operation creates it.
  The principals list matches all the users and groups that have explicitely the permission on the document, either in the local acl or from inheritance.